Though cellular communication has found its place as replacement of wired communication which was used earlier for voice communication.
Now-a-days cellular infrastructure is not only used for voice but also for data.
Due to wide use of internet applications, more and more cellular technologies have been developed to support
high speed internet applications such as LTE, LTE-advanced etc.

Cellular System Architecture

The basic cellular system composed of three elements to provide service to the user(voice or data).
1. Mobile terminal or phone
2. Cellular Tower(i.e. base station)
3. Backbone infrastructure

Different wireless standards have been evolved to support different bandwidths and different coverages
in cellular domain. Mobile phone provides a way to connect with cellular tower.
cellular tower provides a way to make 2-way communication possible by providing connection with the peers(other mobiles) in the
system. Backbone infrastructure includes systems required for base station to base station connectivity as well as
base station to other system connectivity(internet, ISDN, PSTN, other cellular technology based base station) etc.
These connection between the cellular subsystems are referred as interfaces. The figure-1 depicts GSM network architecture
with various system elements and interfaces.

Mobile and base station communicate using radio frequency waves or electro-magnetic waves.
Different RF frequencies are planned based on need of the region. For example in GSM based
cellular technology 900 MHz band is used.
As we know for 2-way communication we need frequency pair one for uplink direction(from mobile to base station)
and the other for downlink direction(from base station to mobile).
In GSM, 890 to 915 MHz is used for uplink and 935 to 960 MHz frequency band is used for downlink.
These bands are further divided into frequency channels with bandwidth as needed to carry
information over the air. This is referred as FDMA(Frequency Division Multiple Access) as explained below.

Access Techniques for resource sharing

The bandwidth from 890 to 915MHz is about 25MHz. This is divided into 124 channels each having 200 KHz of bandwidth.
Further to accommodate more users each frequency channels are assigned eight time slots. These time slots are also assigned to users
which is known as TDMA(Time Division Multiple Access). In GSM cellular technology FDMA/TDMA is used to provide resource allocation to the
users. The other technique known as CDMA is also prevailed in IS-95, CDMA-2000 and WCDMA based cellular technologies where in unique
codes are assigned to both mobile users as well as base stations to differentiate them in the system.
CDMA stands for Code Division Multiple Access.
Refer FDMA vs TDMA vs CDMA for basics and difference between these access techniques.

Cellular Coverage Expansion

Before users are provided with the cellular service, towers or cellular infrastructure need to be installed.
Moreover connectivity between cellular systems using fiber optic or microwave or satellite links need to be in place.
These connectivities are very essential to support long distance call between state to state and country to country.
Let us narrow down our discussion to the cellular network.

In cellular network deployment, area or region is divided into clusters. These clusters are further divided into cells.
Each of these cells are equipped with base stations(BTSs). Shape of the cell is usually hexagonal. Cell size depends largely on
natural terrain as well as man made infrastructures such as buildings etc. Hence the shape is not truly hexagonal.

As frequency is very improtant resource, RF engineers have developed concept called frequency reuse where in same
frequency can be reused in the other cell without any interference.
To avoid any co-channel interference issues, frequency reuse concept is employed in different clusters.
Hence cells in the different nearby clusters or neighbors can use the same frequency channels.
Refer basics of radio frequency planning for more information.

Earlier days transmission power of base station or cellular tower was kept high to reach the
subscribers in the farthest area possible.
Now, to address the demand of increasing number of cellular subscribers(i.e. users) in the urban areas,
cells are divided into narrow cells. The base stations transmit
low power in these narrow cell areas to avoid RF interference related issues.
This concept is known as cell splitting. Figure-2 depicts 3 clusters each cluster having 7 cells.
As shown cell-1 in all of these three clusters can use the same frequency pair.

Handover concept

Handover is the term provided to the movement of mobile phone user from one place to the other place.
Depending upon the speed of the movement there are various different types of handovers (handoffs).
As different cellular systems exist today, concept of handover is not limited within one technology or system.
It has spread between technologies.
Hence GSM to LTE handover, GSM to CDMA handover, CDMA to GSM handover, LTE to GSM handover are to be provided to
support uninterrupted seamless service.

Further to support huge data bandwidth demand from subscribers, cellular offloading concept has been evolved.
In cellular data offloading, data carried by the cellular systems has been transferred to the WLAN based indoor systems
using cellular to WLAN handover concept.

Next Generation Cellular technologies

With the invention of OFDM based multiple carrier concept, more data bits are accommodated over the
carriers and transmitted simultaneously.
Refer SC vs OFDM for the difference between single carrier and multiple carrier based systems.

The cellular wireless technologies such as mobile wimax, LTE and LTE-A use OFDM and OFDMA based techniques
to provide high data rate demand of the users.
Further techniques such as MIMO and beamforming also provide high data rate and better coverage respectively.

The system architectures, access techniques, RF frequencies and bandwidths vary for different wireless technologies as explained here,
but the basic system elements explained in this cellular communication tutorial remains the same.

For further study on cellular communication specific to different wireless standards such as WiMAX, GSM, CDMA, LTE
one can follow links provided below on indoor and outdoor wireless technologies.